The present invention relates to a heat transfer tube for use in heat exchangers of, for example, air conditioners, refrigerators and so forth, and also relates to a method of producing the heat transfer tube. The heat transfer tube of the invention is suited particularly to heat transfer between a single phase flow in the tube and a fluid flowing outside the tube.
Heat exchangers of air conditioners and refrigerators incorporate heat transfer tubes and various types of heat transfer tubes have been proposed, with of these heat transfer tubes have smooth inner surfaces, while other heat transfer tubes have two- or three-dimensionally machined surfaces. For instance, the U. S. Pat. No. 3,768,291 shows a heat transfer tube having two-dimensional ribs formed on the inner surface thereof, and U.S. Pat. No. 3,830,087 discloses a heat transfer tube in which a rolling plug is driven into the tube blank so as to effect a grooving thereby forming primary ribs and then an additional machining is conducted to form secondary grooves, thus providing the tube inner surface with three-dimensional projections.
The heat transfer tubes having two- or three-dimensionally machined inner surfaces encounter the following problems when used for a single phase flow of a fluid. Namely, since the edges of the projections on the tube inner surface are not rounded by are sharp, exfoliation eddy current are formed in the fluid when the fluid turns around the sharp corners or edges, so that a large pressure drop is caused between the inlet and outlet ends of the heat transfer tube, requiring a greater power for driving the fluid through the tube. In addition, the fluid tends to stagnate on the rib surfaces perpendicular to the streamline so that the kinetic energy possessed by the fluid is changed into collision pressure to cause a wear of the ribs during a long uses. As a result, the heights and shapes of the ribs are gradually changed from the optimum design heights and shapes, resulting in a degradation of the heat transfer performance.
In addition, the method for forming ribs by rolling plug essentially requires a troublesome work including the primary grooving and secondary grooving, resulting in a raised production cost of the heat transfer tube.
Accordingly, an object of the invention is to provide a heat transfer tube for single phase flow, having a high heat transfer rate and provided with a highly durable construction of the heat transfer surface, as well as a method which permits the production of such a heat transfer tube at a low cost.
To this end, according to the invention, there is provided a heat transfer tube having projections formed on the inner surface thereof, wherein each projection having a cross-section constituted by smooth curves such as a circle or an ellipse at its bottom and at its any desired height, and wherein the ribs are regularly arranged along spiral curves.